According to prior art, photon counting multi-slit scanning enables X-ray imaging, in particular mammography, with a low X-ray dose and effective rejection of scattered photons. Examples of multi-slit scanning apparatuses are presented in U.S. Pat. Nos. 7,302,031; 7,016,458 and 7,020,237 and also illustrated in FIGS. 1-2. In a typical multi-slit scanning X-ray apparatus, the X-rays are detected by a set of line detectors. A multi-slit collimator is located between the X-ray source and the imaged object. The multi-slit collimator forms a bundle of X-ray beams that matches the pattern of the line detectors. The bundle of X-ray scans through the imaged object, and an image is constructed by accumulation of data from the line detectors. In the apparatus of FIG. 1, for example, the scanning is performed by rotating a scan arm, which is basically a rigid shaft with an X-ray source (110), a multi-slit collimator (120) and a package of line detectors. For example, a human breast may be arranged between a compression paddle (140) and a support above the detector package. A position encoder measures the position of the compression paddle, which is equal to the breast thickness, which is used to optimize a voltage in the X-ray source.
The collimator itself may be a tungsten plate with narrow elongated apertures, referred to as slits. The tungsten plate lacks rigidity in transverse direction, and is therefore arranged in a frame, typically made of carbon fibers, which is reasonably susceptible to X-rays. Different examinations may benefit from different setup of multi-slit collimators, For example, thin collimator slits enable better image resolution than wide slits, but wide slits allow larger X-ray flux and thus shorter total time for image acquisition. U.S. Pat. No. 6,621,891, for example, presents a method and arrangement for controlling collimator slit widths, using two layers of collimators, moving relative to each other.
The collimator has been designed to work in a position which enables imaging of different object thicknesses, e.g. thick breasts as well as thin breasts. It can be easily understood that the collimator impacts image resolution. The X-ray source has a focal spot of a size that is not negligible, which causes blur. The combination of a big focal spot and a collimator far from the object may cause the thin X-ray beams to spread substantially before reaching a small imaged object. FIG. 3 presents an exaggerated illustration of blur due to a large focal spot size and a collimator being placed close the X-ray source. Usually, a collimator, according to prior art, works at only one distance from the X-ray source, due to different magnification factors of the cone beam geometry. Simply displacing the collimator closer to the X-ray source would widen the bundle of beams, and the beams would no longer hit the line detectors. It is possible to use a different collimator at different distance, provided that collimator is resized relative to the original. A collimator may be manually switched by a human operator. The switching is intended for switching between different applications, such as normal breast imaging and biopsy mode. The manual switching is however too slow for a typical screening mammography workflow. The collimator must be attached by a rigid mechanism at high repetitive accuracy. For patient safety, it must be verified or inspected that the collimator is properly attached. Above the multi-slit collimator, the X-ray flux is very high, and the patient would receive an unacceptable dose if X-rays pass outside the collimator slits.
The robustness of the collimator is critical for image resolution, which calls for carefulness in every design of the collimator and surrounding mechanics. The collimator position and slit width shall be invariant to rotation of the X-ray apparatus for acquiring mediolateral oblique view (MLO) images at 45-60 degrees, mediolateral view (ML) images at 90, and 180 degrees images for patients with disabilities. In addition, the space is very limited around the collimator. In mammography, there is virtually no space in front of the collimator, as the patient's breast is imaged very close to the patient's ribs. The space is also limited on the left and right, due to the need of acquiring ML/MLO images close to the patient's shoulder.
The collimator's position must also be possible to calibrate relative the X-ray source and the line detectors. At least, the collimator position must be adjustable essentially orthogonal to the slits, in both ends of the collimator, i.e. a small rotation and translation in a plane mainly transverse the direction of X-rays. In existing techniques, tolerance of adjustment is typically 30 micrometers transverse to the slit direction. In other directions, the tolerances are moderate, but still call for rigidity and calibration. According to prior art, each collimator's position may be calibrated relative a holder, which in turn is rigidly attached to the system.
The collimator may also be displaced between examinations, e.g. according to patent U.S. Pat. No. 7,664,223. According to the apparatus in FIGS. 1-2, the collimator is arranged to be displaced vertically along a scan arm, towards a rest position near the X-ray source. The scan arm is essentially a rigid shaft for carrying the X-ray tube, the collimator and the detector.
There may also be a radiation cover, e.g. according to U.S. Pat. No. 7,440,539, which must be attached in a way that the collimator does not deflect due to the weight of the cover.